CA2397360A1

CA2397360A1 - Stuffing box for progressing cavity pump drive

Info

Publication number: CA2397360A1
Application number: CA002397360A
Authority: CA
Inventors: Vern Arthur Hult
Original assignee: Oil Lift Technology Inc
Current assignee: Oil Lift Technology Inc
Priority date: 2002-08-09
Filing date: 2002-08-09
Publication date: 2004-02-09
Also published as: US7044217B2; US20050011642A1

Abstract

Progressive cavity (PC) pump drive heads require a stuffing box to seal crude oil from leaking onto the ground where the polish rod passes from the crude oil passage in the wellhead to the drive head. Because crude oil typically contains fine sand particles, alignment between the stuffing box and the polished rod is imperfect, and PC drive heads run continuously, it is very difficult make stuffing boxes that last as long as desirable by oil production companies. By using a flexibly mounted standpipe in various configurations, around which is a bearing supported shaft carrying the sealing means, long term durability of the stuffing box can be achieved. By using a pressurization system such that the pressure output exceeds the pressure at the flow line, stuffing box seal life can be extended and external leakage from the stuffing box can be eliminated. A double wall standpipe may be provided in some applications as part of the pressurization system. In some cases pressurization of the stuffing box is sufficiently advantageous that a floating standpipe is not economically warranted.

Description

STUFFING BOX FOR PROGRESSING CAVITY PUMP DRIVE
Field of the Invention The present invention relates generally to improvements in stuffing box configurations for progressive cavity (PC) pump drive head installations.
Background of the Invention Surface drive heads for progressive cavity pumps (wellhead drives) require a stuffing box to seal crude oil from leaking onto the ground where the polished rod passes from the crude oil passage in the wellhead to the drive head.
Due the abrasive sand particles present in crude oil and poor alignment between the wellhead and stuffing box, leakage of crude oil from the stuffing box is common in some applications. This costs oil companies money in service time, down time and environmental clean up. It is especially a problem with heavy crude oil wells in which the oil is often produced from semi-consolidated sand formations since loose sand is readily transported to the stuffing box by the viscosity of the crude oil. It is very difficult to make stuffing boxes that last as long as desirable by oil production companies. Costs associated with stuffing box failures are one of the highest maintenance costs on many wells.
Conventional stuffing boxes are mounted below the wellhead drive.
Conventional stuffing boxes are typically separate from the wellhead drive and are mounted in a wellhead frame such that they can be serviced from below the wellhead drive without removing it. A conventional stuffing box uses braided packing that can be replaced while the polished rod stays inside the stuffing box. Since conventional stuffing boxes seal against the polished rod, which is subject to wear, and due to poor alignment of the polished rod to the stuffing box, leakage becomes somewhat

-2-inevitable. Due to this experience, users tend to expect stuffing box leakage if the stuffing box uses braided packings.
In order to reduce or eliminate the leakage, high-pressure lip seals have been used running against a hardened sleeve rather than against the polished rod.
Grenco Canadian patent 2,095,937 shows a typical stuffing box employing lip seals.
These stuffing boxes are known in the industry as environmental stuffing boxes because they do not leak at all until the lip seals fail. Since these high-pressure lip seals cannot be replaced with the polished rod in place, the wellhead drive must be removed to service the stuffing box. Since the wellhead drive must be removed to service the lip seals, the wellhead frame has been eliminated and the stuffing box is bolted directly to the bottom of the wellhead drive on many drives now being produced. This type of stuffing box directly mounted to the drive head is shown in the above referenced Greco patent.
These types of stuffing boxes are referred to as integral.
Servicing of stuffing boxes is time consuming and difficult. In order to service the environmental or integral stuffing boxes, the drive must be removed which necessitates using a rig with two winch lines, one to support the drive and the other to hold the polished rod. To save on rig time, the stuffing box is typically replaced and the original stuffing box is sent back to a service shop for repair.
Recently, Oil Lift Technology Inc. has introduced top mounted stuffing boxes to the industry, which allow the stuffing box to be serviced without removing the wellhead drive from the well. These types of stuffing box are shown in Hult Canadian patent application 2,350,047 (the "Oil Lift Stuffing Box"). These top mounted stuffing boxes use a flexibly mounted "floating" standpipe around which is a bearing supported shaft carrying the sealing means. Typically the primary rotating seal is braided packing since it has proven to last for a long time when running against the hardened, flexibly mounted standpipe. Because the standpipe floats, it self aligns to the packing, reducing or eliminating run out and leakage compared to conventional stuffing boxes.
In some cases the stuffing box is counter-pressurized by lubricating oil at a higher

-3-pressure than the wellhead pressure such that any leakage past the braided packing goes down the well. In the most difficult applications, the use of pressurized lubricating oil has proven very beneficial in extending seal life, demonstrating many times the stuffing box seal life compared to non-pressurized stuffing boxes.
The present invention relates to improving the performance of the Oil Lift Stuffing Box and to providing a series of stuffing boxes to retrofit to other wellhead drives either above or below the drive head.
Summary of the Invention Hult Canadian patent application 2,350,047 in its entirety is hereby incorporated by reference into this specification.
The present invention relates generally to improvements in stuffing box confrgurations. The present invention also relates generally to improvements in seal configurations for stuffing boxes.
The present invention is applicable to top mounted stuffing boxes, bottom mounted stuffing boxes, integral stuffing boxes and stand-alone stuffing boxes.
Stuffing boxes according to the present invention may either be pressurized or non-pressurized.
Where the stuffing box is pressurized, the pressure may be applied through a fluid medium. The fluid medium may be any suitable liquid or gas. In some applications, the fluid medium is preferably a lubricating fluid such as a lubricating oil so that the fluid medium is available to lubricate stuffing box or drive head components such as seals and bearings.

-4-Where the stuffing box is pressurized, the pressure source may be comprised of any suitable pressure source, including a hydraulic drive for the well, a separate pump, a pressurized chamber such as a chargeable pressure chamber, a pressure-intensifying cylinder, or combinations thereof. The pressure source may also consist of or be comprised of a hydraulic accumulator for maintaining or stabilizing the pressurization of the stuffing box.
Where the stuffing box is pressurized, a pressurization seal is preferably provided for containing or inhibiting the leakage of pressure fluid within or from the stuffing box. The pressurization seal may be comprised of any type of suitable seal, including a labyrinth seal, a braided packing, an O-ring, a lip seal, or combinations thereof. Preferably the pressurization seal is comprised of a braided packing.
Where the stuffing box is pressurized, a circulation path is preferably provided for circulating pressure fluid which does leak within or from the stuffing box. This circulation path may in some applications facilitate lubrication by the pressure fluid of stuffing box or drive head components such as bearings or seals.
Where the stuffing box is non-pressurized, a controlled leakage path is preferably provided for well fluids to prevent or inhibit such fluids from entering the stuffing box bearings or the drive head. The leakage path may comprise one or more holes in components of the stuffing box or the drive head.
Stuffing boxes according to the present invention include a stuffing box seal.
The stuffing box seal may be comprised of any suitable seal, including a braided packing, a lip seal, or a combination of a braided packing and a lip seal.
Preferably the stuffing box seal is comprised of a braided packing or a combination of a braided packing and a lip seal.
Stuffing boxes according to the present invention may utilize a flexibly mounted "floating" standpipe or a rigidly mounted standpipe for improving the performance of

-5-the stuffing box seal. Where a standpipe is utilized, the standpipe may be either a single wall standpipe or a double wall standpipe. A double wall standpipe is useful for facilitating a pressurized stuffing box in which the pressurization seal is comprised of a braided packing or a lip seal.
In order to pressurize the Oil Lift Stuffing Box, a labyrinth seal has been used between the main shaft or the driven gear and the wellhead drive housing. A
labyrinth seal has been used because it is non-wearing, but due to its location in the drive head it is impossible to service without disassembling the drive head. It has also been found that good labyrinth sealing is difficult to achieve due to run out between mating parts and the need for tight tolerances.
In one aspect of the present invention, the need for a labyrinth seal in pressurized stuffing boxes according to preferred embodiments of the invention has been eliminated by use of a double wall standpipe and a set of braided packings located below the first set of braided packings, separated from the first set by a preload spring or a lantern ring. In preferred embodiments, both sets of packings can be serviced in the field without disassembling the drive head.
Another aspect of the present invention is that in some preferred embodiments, two different fluids can be used inside the drive head. Hydraulic pressure, from the hydraulic system driving the drive head, can be used to pressurize the stuffing box.
The lower bearings and gears can be lubricated with gear oil since the stuffing box may be designed so that there is no communication between the hydraulic oil and the gear oil except for leakage across the lower set of packings, which leakage may if desired be designed to be negligible.
In another aspect of the present invention, a non-pressurized stuffing box can be achieved using a flexibly mounted standpipe around which is a bearing supported shaft carrying the sealing means. This configuration can be used for a top mounted stuffing box as part of a drive head or a stand-alone stuffing box that can be retrofitted

-6-to existing drive heads. Since there is no pressurization system, external leakage is preferably provided for to prevent well fluids from entering the drive or stuffing box bearings. Improvements in this system over Hult Canadian patent application 2,350,047 are shown in greater detail with reference to the drawings.
In some cases, it is not economic or practical to provide a pump to pressurize the stuffing box. In these cases, a pressure chamber can be added to the stuffing box so that a pressure fluid is made available at a pressure above the wellhead pressure.
In some cases, hydraulic pressure is readily available to provide for stuffing box pressurization. However, a standpipe system requires a large main shaft and large bearings, which may be too expensive for some applications. In these cases, a bottom-mounted stuffing box with a pressurization system may be an economic solution. This can be done as integral with the drive head or as a separate stuffing box mounted in a wellhead frame.
In another aspect of the present invention, a stuffing box can be constructed with a rigid standpipe with a rotating housing bearingly supported onto the standpipe.
The bearings can be lubricated with the pressurization fluid as it travels into the lower side of the stuffing box seal. This configuration is simpler to construct than a double wall standpipe but it uses more length and does not align the standpipe and the body as well as the double wall standpipe configuration.
Description of the Drawings and of Preferred Embodiments Preferred embodiments of the present invention demonstrating the concepts of the present invention are now described with reference to the appended drawings.
Figure 1 is a cross sectional view of the prior art stuffing box with floating standpipe and labyrinth seal shown as Figure 6 in Hult Canadian patent application 2,350,047.

-7-Figure 2 is a cross sectional view of the prior art stuffing box with floating standpipe but no pressurization system, shown as Figure 8 in Hult Canadian patent application 2,350,047.
Figure 3 is a cross sectional view of the prior art stuffing box pressurized from the hydraulic system, shown as Figure 9 in Hult Canadian patent application 2,350,047.
Figure 4 is a cross sectional view of the preferred embodiment of a stuffing box with a floating single wall standpipe but without a pressurization system. It is an improvement compared to Figure 2 since it eliminates the lip seals below the lantern ring, using braided packings instead. Self lubricating braided packings are used below the lantern ring because the previously shown lip seals could not be lubricated and wore very quickly. Additionally, a high pressure lip seal can be fitted above the packings to ensure zero leakage until it wears out. Since the standpipe self aligns to the rotating seals, this lip seal configuration has life advantages over the configuration used in Grenco Canadian patent 2,095,937.
Figure 5 is a cross sectional view of a preferred embodiment of a stuffing box using a floating double wall standpipe pressurization system. The need for a labyrinth seal as shown in Figures 1 and 3 has been eliminated by use of a double wall standpipe and a set of braided packings located below the first set of braided packings, separated from the first set by a preload spring or a lantern ring.
Both sets of packings can be serviced in the field without disassembling the drive head.
In the event of wear to the standpipe, it can be removed from the top of the drive without removing the drive from the well.
Figure 6 is a preferred embodiment of a stand-alone stuffing box mounted in a wellhead frame using a floating double wall standpipe pressurization system.
The stuffing box would be pressurized off the hydraulic system that is powering the _g_ wellhead drive. The pressure from the hydraulic system is preferably reduced down to about 150 psi above the wellhead pressure by the built in pressure-reducing valve.
Figure 7 is a preferred embodiment of a stand-alone stuffing box mounted in a wellhead frame using a floating double wall standpipe pressurization system.
In this case, the pressurization source is a pressure-intensifying cylinder built below the stuffing box, surrounding the polished rod. Grease or oil under pressure is put into the upper chamber to push the piston down. Wellhead pressure pushes on the bottom of the piston, urging the piston upward. Since the piston area on the wellhead side is larger than on the stuffing box side, oil or grease feeds into the stuffing box at higher pressure than the wellhead pressure. By mounting the cylinder between the stuffing box and the wellhead, heat is conducted into the cylinder to prevent the cylinder from freezing. There are no separate fluid lines to freeze off in cold weather. It will be appreciated that this pressurization system can be used whether the stuffing box is a stand-alone version or is built into the drive head.
Figure 8 is a preferred embodiment of a stand-alone stuffing box mounted in a wellhead frame using a floating single wall standpipe without a pressurization system.
Space is often a constraint when retrofitting stuffing boxes to existing equipment. In general terms, the sealing system is equivalent to Figure 4, except the lantern ring and extra set of packings in Figure 4 is replaced by a leakage path out of the housing and a lip seal between the main shaft and the housing to keep well fluids out of the stuffing box bearings.
Figure 9 is a preferred embodiment of a stand alone stuffing box constructed with a rigid single wall standpipe with a rotating housing bearingly supported onto said standpipe. The bearings can be lubricated with the pressurization fluid as it travels into the lower side of the stuffing box seal. This configuration is simpler to construct than a double wall standpipe but it uses more length and does not align the standpipe and the body as well as the double wall standpipe configuration.
_g_ Figure 10 is a preferred embodiment of a wellhead drive with an integral stuffing box mounted on the bottom of the drive with a pressurization system. In some cases, hydraulic pressure is readily available to provide for stuffing box pressurization.
However, the standpipe system requires a large main shaft and large bearings, which may be too expensive for some applications. In these cases, a bottom-mounted stuffing box with pressurization is an economic solution. This can be done as integral with the drive head or as a separate stuffing box mounted in a wellhead frame.
In this preferred embodiment, there are two sets of packings separated by a packing preload spring that acts as a lantern ring. The packings run on a hard sleeve that is supported on an extension of the main shaft. The main shaft is supported by bearings and the stuffing box is fitted to the wellhead drive with a pilot diameter to align the rotating shaft with the stuffing box seals. Although alignment is not as good as with a floating standpipe, this is a cost effective solution suitable in conditions where stuffing box wear is not severe.
Figure 11 is a stand-alone stuffing box similar to and using the same principles as the integral stuffing box shown in Figure 10.